Liquid natural gas qualifies as a desirable alternative fuel for internal combustion engines. A major problem associated with the use of liquid natural gas as a fuel for internal combustion engines is that liquid natural gas is a mixture of about 90 to 95% methane with higher hydrocarbons, the principal higher hydrocarbon being ethane, usually in the range of from about 4% to about 7%.
The hydrocarbons higher than methane create several problems for the utilization of liquid natural gas as a fuel for internal combustion engines. First, the higher hydrocarbons have lower auto ignition temperatures than methane.
______________________________________ Critical Auto Ignition Component Compression Ratio Temperature ______________________________________ methane 13.0 540.degree. C. ethane 9.8 515.degree. C. propane 8.8 450.degree. C. butane 5.3 405.degree. C. pentane 3.5 260.degree. C. ______________________________________
The composition of natural gas and, therefore, the percentage of higher hydrocarbons varies widely dependent on the source. Such variation in composition denies engine manufacturers the opportunity to maximize engine designs. The higher hydrocarbons in the liquid natural gas fuel can preignite and result in preignition of the methane. This causes knock, hot spots and eventual engine failure.
Many processes have been devised for the cryogenic separation of heavier components from a natural gas stream and disposing the waste "dirty" methane stream usually by returning it to the pipeline. Among these are U.S. Pat. Nos. 4,072,485 to Becdelievre, et al.; 4,022,597 to Bacon; 3,929,438 to Harper; 3,808,826 to Harper, et al.; Re. 29,914 to Perret; Re 30,085 to Perret; 3,414,819 to Grunberg, et al.; 3,763,658 to Gaumer, Jr., et al.; 3,581,510 to Hughes; 4,140,504 to Campbell, et al.; 4,157,904 to Campbell, et al.; 4,171,964 to Campbell, et al.; 4,278,457 to Campbell, et al.; 3,932,154 to Coers, et al.; 3,914,949 to Maher, et al. and 4,033,735 to Swenson.
Such prior art processes for separation of heavier components utilize complex heat exchange schemes usually involving fractionation in a distillation column. Exemplary of such processes is U.S. Pat. No. 4,738,699 to Apffel. The Apffel patent discloses a method for use of a mixed refrigeration stream for removing higher hydrocarbons from methane of a natural gas stream. The mixed refrigeration system uses two-phase flow for refrigeration to facilitate separation of the hydrocarbon components, such as ethane, propane and heavier gases from methane and lighter constituents of the natural gas stream. The separation process is accomplished in two stages. First, the inlet gas stream is cooled in exchange with a refrigerant and residue gas and partially condensed. Second, the condensed mixture and the vapor stream are fed to a fractionation tower, where the desired hydrocarbons are separated from methane and lighter gases using indirect heat exchange with the mixed refrigerant, and a slip stream from the initial feed stream, alternately to provide the energy for distillation.
It is a principle object of the present invention to provide a simple means for providing a purified methane product suitable for use in internal combustion engines utilizing liquid nitrogen as the driving force for the purification and the liquefication of the natural gas.